For production of nodular cast iron. Fe-Si-Mg alloy or such alloy added with a small amount of RE or rare earth elements is most commonly used as the spheroidization agent in industrial applications. Also, the open ladle addition process or the so-called sandwich process is most commonly used as the spheroidization process.
To the end of improving the graphitization capability of molten metal which has been temporarily reduced by addition of Mg or Mg-RE alloy, an inoculation agent such as various Si alloys or graphite-base substances has been conventionally inoculated into the ladle and/or the flow of the molten metal as it is poured into a mold. However, in industrial applications, since simply performing inoculation after a spheroidization process cannot entirely eliminate the formation of cementite, a heat treatment is required in order to decompose the cementite formation.
Hence, unfavorable consequences such as the increase in the cost and time required for production are inevitable.
The process of producing nodular cast iron which was the subject of a preceding patent application (Japanese patent application No. 61-144591) can produce a favorable chill prevention effect with respect to thin-shell cast iron products, but the present invention has its aim to further improve this prior invention by adding bismuth (Bi) as a graphite atomization agent.
The efficacy of Bi addition to atomize graphite has already been reported, for instance, in AFS Internar., Cast Metals, J, 7(1982), 3, S, 22/31 and FONDERIE BELGE 52 (1982) Nr, 2, S, 5/18, and inoculating agents containing Bi such as SPHERIX (trade name) are commercially available.
However, according to the present invention, through the synergetic effect of processing the molten metal with a graphitization agent including SiC or CaC.sub.2 as a major ingredient and of adding Bi, the promotion of graphitization and the increase in the number of graphite nodules, which are both important for the production of high-quality thin-shell cast iron products, can be accomplished. For example, according to the results of a comparison test conducted with respect to the Y-blocks, which were cast from different kinds of nodular cast iron and are each 25 mm in thickness, by taking into account only the graphite nodules having 8 micrometers or greater in diameter, it was observed that, whereas the number of graphite nodules was 300/mm.sup.2 according to the process of producing nodular cast iron disclosed in Japanese patent application No. 61-144591, and this number was no more than 300/mm.sup.2 when Bi was simply added, the present invention was able to increase this number to 600/mm.sup.2.